This invention relates generally to aircraft and their component systems, and, more particularly, to improved high-performance aircraft systems capable of high-altitude stationkeeping within tight altitude and perimeter boundaries for extended periods of time.
A worldwide expansion in the demand for communication bandwidth is driving up the bandwidth requirements between satellites and ground-stations. One way to increase this satellite-to-ground bandwidth is to interpose one or more high-altitude platforms (HAPs) configured for relaying signals between the two. A HAP allows for lower power transmissions, narrower beamwidths, as well as a variety of other advantages that provide for greater bandwidth. However, due to a demanding set of design requirements, years of design efforts at creating highly effective HAPs are only now beginning to come to fruition.
In particular, it is desirable to have a stratospheric aircraft, capable of carrying a significant communications payload (e.g., a payload of more than 100 kg that consumes more than 1 kw of electric power), that can remain aloft for days, weeks or even months at a time. This flight capability will preferably be maintainable even in zero or minimum sunlight conditions where solar power sources have little functionality. Also, the aircraft is preferably remotely pilotable to limit its weight and maximize its flight duration.
The communications payload preferably is configured to view downward over a wide, preferably unobstructed field of view. The aircraft will preferably be capable of relatively high-speed flight that is adequate to travel between its station and remote sites for takeoff and/or landing to take advantage of benign weather conditions. At the same time, the aircraft preferably is capable of maintaining a tight, high-altitude station in both high-wind and calm conditions, thus requiring relatively high-speed and relatively low-speed flight, and a small turning radius while maintaining the payload""s downward-looking (and preferably upward-looking for some embodiments) view. To meet these stringent design specifications, the performance of the aircraft""s power system, flight control system and airframe configuration and are all preferably improved over prior practice.
Power Systems
Conventional aircraft are typically powered using aviation fuel, which is a petroleum-based fossil fuel. The prior art mentions the potential use of liquid hydrogen as a fuel for manned airliners and supersonic stratospheric flight. There is also 25-year-old prior art mentioning the possibility of using liquid hydrogen as fuel for a stratospheric blimp.
U.S. Pat. No. 5,810,284 (the ""284 patent), which is incorporated herein by reference, discloses an unmanned, solar-powered aircraft that significantly advanced the art in long-duration, stratospheric aircraft. It flies under solar power during the day, and stores up additional solar power in a regenerative fuel cell battery for use during the night to maintain its station. The fuel cell battery is a closed system containing the gaseous elements of hydrogen and oxygen that are dissociated from, and combined into, water.
The aircraft disclosed in the ""284 patent is an unswept, span-loaded, flying wing having low weight and an extremely high aspect ratio. Multiple electric engines are spread along the wing, which is sectionalized to minimize torsion loads carried between the sections. Most or all of the sections contain a hollow spar that is used to contain the elements used by the fuel cell. Large fins extend downward from inner ends of the sections. The wings contain two-sided solar panels within transparent upper and lower surfaces to take maximum advantage of both direct and reflected light.
The above-described technologies cannot provide for long-duration, high-altitude flights with tight stationkeeping when the available solar power is highly limited.
Flight-Control Components
Various components are known for use in controlling flight. Each component has unique advantageous and disadvantageous characteristics.
Many present-day small aircraft and some sailplanes use simple flaps to increase camber and obtain higher lift coefficients, and hence, adequate lift at lower speeds. Such flaps are typically retracted or faired to reduce drag during high-speed flight, and also during turbulence to reduce the maximum G loads that the wing will then experience. An important characteristic of the use of flaps, or of the use of highly cambered airfoils designed for high lift, is that the extended flap or highly cambered airfoil provides the wing with a large negative pitching moment. This affects both overall vehicle stability and the wing""s torsional twisting. Indeed, for high aspect-ratio wings, the twist at the wing""s outer portions due to a negative pitching moment can pose severe structural and flight control problems.
Airliners use both leading edge slats and sophisticated flaps, such as slotted or Fowler flaps, to widen their speed range. Small planes employ slats that open automatically when needed. Hang gliders have employed flexible airfoil tightening to decrease camber for high-speed flight. Some work has been done with flexible flap material that unrolls and pulls back from the rear of the wing. Some aircraft feature wings characterized by a sweep that can be varied in flight, even turning the entire wing so that it is not perpendicular to the flight direction during high-speed flight.
For maintaining low-speed flight without stalling, large solid or porous surfaces that hingedly swing up from a wing top in low-speed flight to potentially stabilize vortices immediately behind them, are known. This might provide an increased lift coefficient before stall is reached. Various vortex generators and fences are used to delay the onset of a stall or to isolate the portion of a wing that is stalled. Furthermore, various stall warning/actuators allow aircraft to operate relatively close to their stall speed. Additionally, some combinations of airfoils and wing configurations feature gentle stalls and so the vehicle can be operated at the stall edge without abrupt drag increases or lift decreases during the onset of a stall. Experimental aircraft have even employed rotary devices to permit low-speed flight, with mechanisms that restrict rotary moment and decrease drag or potentially augment lift when at higher speeds the wing provides the main lift. Many of the above mechanisms provide this increased low-speed control at the expense of weight and reliability.
In some high-tech aircraft, highly-active control is used to maintain stable operation over a wide range of speeds and orientations. This emulates the flying characteristics of natural fliers that change wing and airfoil geometry. In aircraft, such systems are complex, potentially heavy, and expensive, as well as fault-intolerant.
Airframe Configuration
The requirements for wide speed range, low power, light weight, unimpeded communications platform view, simplicity, and reliability present significant tradeoff challenges. A highly cambered airfoil helps with lowering minimum flight speed, but is accompanied by a large negative pitching moment that impacts the aeroelastic effects of wing twist.
Furthermore, there is an inherent relationship between an aircraft""s overall airframe geometry and the design of its airfoils and control surfaces. Typical aircraft offset negative (i.e., nose-down) pitching moments through the use of tail moments (i.e., vertical forces generated on the empennage with a moment arm being the distance from the wing to the empennage) or through the use of a canard in front of the wing that, for pitch stability, operates at a higher lift coefficient than the wing and stalls earlier. Tails mounted in the up-flow of wingtip vortices can be much smaller than tails positioned in the wing downwash, but there are structural difficulties in positioning a tail in the up-flow.
Commercial airliners address the high coefficient of lift (CL) requirements for landing and takeoff with a complex array of slats and flaps that are retracted during high-speed flight to lower drag and gust-load severity. A rigid wing structure, and pitch controllability from the tail""s area and moment arm, permit this approach. However, this approach is contrary to the requirement that the present aircraft carry fuel adequate to last for extended periods of time, and still be economical.
The very special requirements and technological challenges for the aircraft of the present invention have not been met by existing aircraft designs. Accordingly, there has existed a definite need for a lightweight aircraft capable of both stationkeeping and flight over a wide range of speeds, that consumes low levels of power for an extended period of time, that supports a communications platform with a wide, unobstructed view, and that is characterized by simplicity and reliability. Embodiments of such an invention can serve as high altitude platforms. Embodiments of the present invention satisfy various combinations of these and other needs, and provide further related advantages.
The present invention provides aircraft, aircraft components and aircraft subsystems, as well as related methods. Various embodiments of the invention can provide flight over a wide range of speeds, consuming low levels of power for an extended period of time, and thereby supporting a communications platform with an unobstructed downward-looking view, while and having simplicity and reliability.
In one variation, a wing of the invention is characterized by having adequate camber to achieve a lift coefficient of approximately 1.5 at the Reynolds number experienced by sailplanes or flexible-winged stratospheric aircraft. The wing defines a leading edge and a trailing edge, and the trailing edge includes either a reflexed portion or a trailing edge flap that can extend upward. Either the reflexed portion or the flap is configured to provide the wing with a pitching moment greater than or equal to zero in spite of the camber. This feature advantageously allows for low-speed flight with a flexible wing in many embodiments.
This feature is augmented by an extendable slat at the leading edge of the wing. These features, in combination, provide for an excellent coefficient of lift of the wing, typically increasing it by more than 0.3, and preferably by 0.4 or more, at airspeeds just above the stall speed. Using its retractability, the slat can become part of the wing""s airfoil that is otherwise defined by the wing""s camber. Slats are convenient because they have a negligible or beneficial effect on a wing""s pitching moment. While flaps might help increase the CL more than slats, they do so at the cost of a big increase in negative pitching moment that potentially requires heavy, drag-producing countermeasures for compensation.
In another variation of the invention, an aircraft comprises a flying wing extending laterally between two ends and a center point, substantially without a fuselage or an empennage. The wing is swept and has a relatively constant chord. The aircraft also includes a power module configured to provide power for the aircraft, and a support structure including a plurality of supports, where the supports form a tetrahedron. This tetrahedron has comers in supportive contact with the wing at structurally stiff or reenforced points laterally intermediate the center point and each end. The tetrahedron also has a comer in supportive contact with the wing""s center point, which is also structurally stiff or reenforced. Advantageously, the flying wing is configured with a highly cambered airfoil and with reflex at a trailing edge. The wing is also configured with slats. These features provide many embodiments with the capability of high-altitude flight with a wide range of speeds.
A third variation of the invention is an aircraft, and its related power system, for generating power from a reactant such as hydrogen. The power system includes a fuel cell configured to generate power using a gaseous form of the reactant, the fuel cell being configured to operate at a power-generation rate requiring the gaseous reactant to be supplied at an operating-rate of flux. The power system also includes a tank configured for containing a liquid form of the reactant, wherein the tank includes a heat source for increasing a boiling-rate of the reactant. The tank is configured to supply its reactant to the fuel cell at a rate determined by the boiling-rate of the reactant, and the heat source is configured to increase the boiling rate of the reactant to a level adequate for supplying the resulting gaseous reactant to the fuel cell at the operating-rate of flux. An advantage of such an aircraft is that it provides for a minimized system weight, volume and complexity, while not excessively sacrificing power generation.
In a fourth variation of the invention, the power system of the third variation includes a tank that comprises an inner aluminum tank liner having an outer carbon layer, an outer aluminum tank liner having an outer carbon layer, and connectors extending between the inner and outer aluminum tank liners to maintain the aluminum tank liners"" relative positions with respect to each other. The volume between the inner and outer tank liners is evacuated to minimize heat transfer between the contents of the tank and the outside environment. The connectors between the inner and outer layers are configured with holes in their walls to minimize direct heat-conduction between the contents of the tank and the outside environment.
In a fifth variation of the invention, an aircraft includes a hydrogen source, an oxygen source and a fuel cell configured to combine hydrogen from the hydrogen source and oxygen from the oxygen source to generate power. The fuel cell is preferably configured to combine the hydrogen and the oxygen at less than one atmosphere of pressure, and more preferably at roughly 2-3 psia. This advantageously allows stratospheric flight with simpler fuel cell technology.
Preferred embodiments of the above aspects of the invention, and various combinations of their features, provide for unmanned aircraft capable of flying in the stratosphere, in a stationkeeping mode, carrying a payload of more than 100 kg that consumes more than 1 kw of electric power, and remaining aloft for a significant period of time while being able to operate from a remote site where takeoff/landing weather is benign.
Other features and advantages of the invention will become apparent from the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.